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9 Control Abstraction

Sample run of chapter's interpreter examples

• 9.1 Reducing code duplication
• 9.2 Simplifying client code
• 9.3 Currying
• 9.4 Writing new control structures
• 9.5 By-name parameters
• 9.6 Conclusion

9.1 Reducing code duplication

// In file control-abstraction/Files1.scala

  object FileMatcher {
    private def filesHere = (new java.io.File(".")).listFiles

    def filesEnding(query: String) =
      for (file <- filesHere; if file.getName.endsWith(query))
        yield file
  }


// In file control-abstraction/Files1.scala

  def filesContaining(query: String) =
    for (file <- filesHere; if file.getName.contains(query))
      yield file


// In file control-abstraction/Files1.scala

  def filesRegex(query: String) =
    for (file <- filesHere; if file.getName.matches(query))
      yield file


  def filesMatching(query: String, method) =
    for (file <- filesHere; if file.getName.method(query))
      yield file


// In file control-abstraction/Files2.scala

  def filesMatching(query: String,
      matcher: (String, String) => Boolean) = {

    for (file <- filesHere; if matcher(file.getName, query))
      yield file
  }


// In file control-abstraction/Files2.scala

  def filesEnding(query: String) =
    filesMatching(query, _.endsWith(_))

  def filesContaining(query: String) =
    filesMatching(query, _.contains(_))

  def filesRegex(query: String) =
    filesMatching(query, _.matches(_))


  (fileName: String, query: String) => fileName.endsWith(query)


// In file control-abstraction/Files.scala

  object FileMatcher {
    private def filesHere = (new java.io.File(".")).listFiles

    private def filesMatching(matcher: String => Boolean) =
      for (file <- filesHere; if matcher(file.getName))
        yield file
  
    def filesEnding(query: String) =
      filesMatching(_.endsWith(query))
  
    def filesContaining(query: String) =
      filesMatching(_.contains(query))
  
    def filesRegex(query: String) =
      filesMatching(_.matches(query))
  }


  

9.2 Simplifying client code

  def containsNeg(nums: List[Int]): Boolean = {
    var exists = false
    for (num <- nums)
      if (num < 0)
        exists = true
    exists
  }


  scala> containsNeg(List(1, 2, 3, 4))
  res0: Boolean = false

  scala> containsNeg(List(1, 2, -3, 4))
  res1: Boolean = true


  def containsNeg(nums: List[Int]) = nums.exists(_ < 0)


  scala> containsNeg(Nil)
  res2: Boolean = false

  scala> containsNeg(List(0, -1, -2))
  res3: Boolean = true


  def containsOdd(nums: List[Int]): Boolean = {
    var exists = false
    for (num <- nums)
      if (num % 2 == 1)
        exists = true
    exists
  }


  def containsOdd(nums: List[Int]) = nums.exists(_ % 2 == 1)


  

9.3 Currying

  scala> def plainOldSum(x: Int, y: Int) = x + y
  plainOldSum: (x: Int, y: Int)Int

  scala> plainOldSum(1, 2)
  res4: Int = 3


  scala> def curriedSum(x: Int)(y: Int) = x + y
  curriedSum: (x: Int)(y: Int)Int

  scala> curriedSum(1)(2)
  res5: Int = 3


  scala> def first(x: Int) = (y: Int) => x + y
  first: (x: Int)Int => Int


  scala> val second = first(1)
  second: Int => Int = <function1>


  scala> second(2)
  res6: Int = 3


  scala> val onePlus = curriedSum(1)_
  onePlus: Int => Int = <function1>


  scala> onePlus(2)
  res7: Int = 3


  scala> val twoPlus = curriedSum(2)_
  twoPlus: Int => Int = <function1>

  scala> twoPlus(2)
  res8: Int = 4


  

9.4 Writing new control structures

  scala> def twice(op: Double => Double, x: Double) = op(op(x))
  twice: (op: Double => Double, x: Double)Double

  scala> twice(_ + 1, 5)
  res9: Double = 7.0


// In file control-abstraction/WithPrintWriter1.scala

  def withPrintWriter(file: File, op: PrintWriter => Unit) = {
    val writer = new PrintWriter(file)
    try {
      op(writer)
    } finally {
      writer.close()
    }
  }


// In file control-abstraction/WithPrintWriter1.scala

  withPrintWriter(
    new File("date.txt"),
    writer => writer.println(new java.util.Date)
  )


  scala> println("Hello, world!")
  Hello, world!


  scala> println { "Hello, world!" }
  Hello, world!


  scala> val g = "Hello, world!"
  g: String = Hello, world!

  scala> g.substring { 7, 9 }
  <console>:1: error: ';' expected but ',' found.
         g.substring { 7, 9 }
                        ^


  scala> g.substring(7, 9)
  res12: String = wo


// In file control-abstraction/WithPrintWriter2.scala

  def withPrintWriter(file: File)(op: PrintWriter => Unit) = {
    val writer = new PrintWriter(file)
    try {
      op(writer)
    } finally {
      writer.close()
    }
  }


// In file control-abstraction/WithPrintWriter2.scala

  val file = new File("date.txt")

  withPrintWriter(file) { writer =>
    writer.println(new java.util.Date)
  }


  

9.5 By-name parameters

  withPrintWriter(file) { writer =>
    writer.println(new java.util.Date)
  }


// In file control-abstraction/Assert.scala

  var assertionsEnabled = true

  def myAssert(predicate: () => Boolean) =
    if (assertionsEnabled && !predicate())
      throw new AssertionError


  myAssert(() => 5 > 3)


  myAssert(5 > 3) // Won't work, because missing () => 


// In file control-abstraction/Assert.scala

  def byNameAssert(predicate: => Boolean) =
    if (assertionsEnabled && !predicate)
      throw new AssertionError


  byNameAssert(5 > 3)


  def boolAssert(predicate: Boolean) =
    if (assertionsEnabled && !predicate)
      throw new AssertionError


  boolAssert(5 > 3)


  scala> val x = 5
  x: Int = 5

  scala> var assertionsEnabled = false
  assertionsEnabled: Boolean = false

  scala> boolAssert(x / 0 == 0)
  java.lang.ArithmeticException: / by zero
    ... 33 elided


  scala> byNameAssert(x / 0 == 0)


  

9.6 Conclusion